1. Field of the Invention
The present invention relates to a light control device, and more particularly, to a light control device using a dielectric anisotropic substrate.
2. Description of the Related Art
Light control devices in which a signal electrode and a ground electrode are formed on a substrate formed of a dielectric anisotropic material, a high-frequency signal is applied to the signal electrode, and the high-frequency signal is propagated along the signal electrode are generally used. Examples of the light control devices include an optical modulator in which a Mach-Zehnder waveguide having a Y axis as a light-propagating direction is formed in a wafer substrate cut by a plane having the X axis of LiNbO3 crystal as a normal vector and a signal electrode and a ground electrode for modulating an optical wave propagated in the waveguide are formed on the substrate, or a light control device performing a modulation operation and an optical path switching operation by interaction of an optical wave and an electric field. Examples of the light control device switching a signal line with an optical waveguide include a light control device using a directional coupler as shown in Patent Literature 2, a light control device in which a large difference in refractive index is provided and light is concentrated on an arm having a high refractive index as shown in Patent Literature 3, a light control device using the total reflection as shown in Patent Literature 4, and a light control device using a Mach-Zehnder interferometer as shown in Patent Literature 5.
In such light control devices, since the dielectric constant is different between the light-propagating direction (Y axis) and a direction (Z axis) perpendicular to the light-propagating direction, an impedance mismatch occurs between the signal electrode along the Y axis direction and the signal electrode along the Z axis direction with the same electrode structure. The impedance mismatch causes an electroreflection phenomenon in a high-frequency area, thereby deteriorating the transmittance of the high-frequency signal.
To solve the above-mentioned problem, Patent Literature 1 discloses that an electrode structure having the middle value between the impedance values before and after it is bent is formed in the middle of the signal electrode which is bent at a right angle.
However, in the electrode structure disclosed in Patent Literature 1, the impedance gradually varies and thus it can be expected to reduce the electroreflection phenomenon slightly, but the impedance rapidly varies locally and the electroreflection phenomenon occurs, whereby a satisfactory loss reduction effect cannot be expected. In addition, the electrode structure for adjusting the impedance is a structure in which a plane of at least one of the signal electrode and the ground electrode opposed to the other electrode has a corner due to the gradual adjustment thereof. The structure having this corner causes the deterioration in high-frequency characteristic.
Patent Literature 1: JP2005-37547A
Patent Literature 2: Japanese Patent No. 3139009
Patent Literature 3: Japanese Patent No. 2746216
Patent Literature 4: Japanese Patent No. 2583480
Patent Literature 5: JP1995-49511A